The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads, is a commercially significant application of polymer film, including generically, polyethylene. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods. Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The roll of film so used may be installed on a hand-held wrapping tool for ease of use by the operator.
Some of the properties desired of a stretch wrap film often include: good cling or cohesion properties, good transparency or opacity, low stress relaxation with time, high resistance to transverse tear when under machine direction tension, producible in thin gauges, low specific gravity, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, high modulus of elasticity and high puncture resistance.
Physical properties which are particularly significant for the successful use of thermoplastic films in stretch wrap applications include their puncture resistance, their elongation characteristics, their toughness, and their resistance to tearing while under tension. In general, tensile toughness is measured as an area under a stress-strain curve developed for a thermoplastic film, and it may be considered as the tensile energy absorbed, expressed in units of ft.-lbs./cu.in. to elongate a film to break under tensile load. In turn, this toughness characteristic is a function of the capacity of such films to elongate. The process of stretching the film decreases that capacity. Accordingly, the stretch wrap process will decrease the toughness of the film while it is in its stretched condition as an overwrap as compared to unstretched counterparts, including such materials as shrink wrap. Generally this loss of toughness is proportional to the amount of stretch imparted to the film as it is overwrapping a load of goods.
As hereinabove indicated, the resistance to tear characteristic of such films will obviously be an important physical characteristic for stretch wrap applications since if the edge of the stretch film roll is nicked, abraded or in any way weakened before stretching or during the stretching operation, premature tearing of the film will usually occur during wrapping or subsequent handling of the load of goods.
Some resins which have been used in the fabrication of stretch wrap film are polyethylene, polyvinylchloride and ethylene vinyl acetate. A fairly recent development has been the utilization of linear low density polyethylene (LLDPE) in the manufacture of stretch wrap film, e.g., as described in U.S. Pat. Nos. 4,399,180, 4,418,114 and 4,518,654, the contents of which are incorporated by reference herein. The excellent toughness and puncture resistance properties of LLDPE makes it an excellent resin for such an application. LLDPE and methods for its manufacture are described in, among others, U.S. Pat. Nos. 3,645,992, 4,076,698, 4,011,382, 4,163,831, 4,205,021, 4,302,565, 4,302,566, 4,359,561 and 4,522,987. In general, films fabricated from the typical LLDPE resins of commerce possess little or no cling property on either surface thereof in the absence of added cling agent. LLDPE films possessing an inherent cling property wherein the LLDPE possesses a relatively high level of n-hexane extractables are disclosed in U.S. Pat. No. 5,273,809, Application Serial No. 07,985,042, having an issue date of Dec. 28, 1993. U.S. Pat. No. 5,273,809 is a divisional of Application Serial No. 07/903,295. filed on Jun. 24, 1992, which is a continuation-in-part of Application Serial No. 07/296,930, filed on Jan. 12, 1989, which is a continuation of Application Serial No. 07/039,892, filed on Apr. 17, 1987, the contents of each of which are incorporated by reference in their entirety.
Other thermoplastic films are known in the art. U.S. Pat. No. 4,311,808 describes a cling film containing a homogeneous mixture of polyisobutylene, ethylene-propylene and a low density polyethylene.
Another multilayer composite film is found in U.S. Pat. No. 4,364,981 in which a core or intermediate layer of low pressure low density polyethylene (LPLDPE) and skin layers of high pressure low density polyethylene (HPLDPE) are taught. The LPLDPE described within U.S. Pat. No. 4,364,981 is actually an LLDPE, as may be seen by reference to the process for making LLDPE of U.S. Pat. No. 4,011,382 disclosed at column 1, lines 52-55. The films disclosed are said to be useful in forming bags such as trash bags. No disclosure of possible utility as a stretch wrap is found within this patent.
U.S. Pat. No. 4,367,256 describes a cling wrap plastic film based on a blend of high pressure low density polyethylene (HPLDPE) and LLDPE in which the latter resin represents from 5-16 weight percent of the total. In one embodiment, this film is sandwiched between two HPLDPE films.
U.S. Pat. No. 4,399,173 describes a multilayer plastic film suitable for use in packaging and in the forming of refuse bags, which is said to be free of melt fracture. The film disclosed therein includes a core layer of low melt index, low pressure, low density polyethylene and two outer layers of a high melt index, low pressure, low density polyethylene. It is disclosed that the preferred method of forming the films of U.S. Pat. No. 4,399,173 is by tubular blown film extrusion.
U.S. Pat. Nos. 4,399,180 and 4,418,114 describe a coextruded composite stretch wrap film in which an LLDPE core layer is surfaced with HPLDPE skin layers.
U.S. Pat. No. 4,511,609 discloses a multilayer film for use in the manufacture of garbage bags wherein the film comprises a first outer layer of LLDPE, a second outer layer of HPLDPE and a intermediate layer also of HPLDPE or a blend of LLDPE and HPLDPE. No statement as to whether the films disclosed have any possible utility as stretch wraps is made within this patent.
In the one-sided cling stretch wrap film of U.S. Pat. No. 4,518,654, layer A of the disclosed A/B structure is fabricated from a resin possessing an inherent cling property and/or a cling property resulting from the incorporation of a cling additive therein. Layer A is coextruded with layer B, which is fabricated from a resin exhibiting little if any cling. Layer B further exhibits a slide property when in contact with a layer of itself with relative motion therebetween. Layer B can contain an anti-cling additive at a level of from about 0.05 to about 2.0 weight percent of the resin component of the layer, and can consist of such materials as crystalline and amorphous silicate. In each of the two working examples of this patent, layer A is an LLDPE film containing a cling additive, namely, polyisobutylene, with synthetic sodium silicate particles being uniformly incorporated throughout layer B of the film of Example 1 and amorphous silica particles being uniformly incorporated throughout layer B of the film of Example 2. Layer B, the layer providing the noncling surface of the films of Examples 1 and 2 is HPLDPE. Improved one-sided cling/one-sided slip stretch wrap films are disclosed in U.S. Pat. No. 4,996,054.
U.S. Pat. No. 4,565,720 discloses three layer packaging bags in which an intermediate layer comprising a mixture of LLDPE and high density polyethylene (HDPE) is taught. Outer layers of the bags are made of HPLDPE or a mixture of HPLDPE and an ethylene-vinyl acetate copolymer. The structures disclosed are not cited as being useful in forming stretch wrap films.
A multilayer film in which the intermediate layer comprises LLDPE and HDPE and the outer layers comprise an LLDPE and at least one such outer layer further comprises an HPLDPE is disclosed in U.S. Pat. No. 4,574,104. One film disclosed therein is a three-layer structure in which the intermediate layer comprises LLDPE, HPLDPE and HDPE. Packaging bags are the chief utility cited for these multilayer films. No disclosure of utility as a stretch wrap is made.
In U.S. Pat. No. 4,657,811, a three-layer plastic film suitable for use in trash bag manufacturing is disclosed wherein an intermediate layer of foamed HPLDPE is disposed between outer layers of LLDPE. Again, the films disclosed are not cited as finding utility as stretch wrap films.
U.S. Pat. No. 4,996,094 discloses a stretch wrap polyolefinic film having one surface with cling properties and the other surface having noncling or slip properties. A high molecular weight cling additive is employed to reduce additive migration and transfer from the cling surface of the film.
U.S. Pat. No. 5,019,315 discloses multilayer coextruded thermoplastic stretch wrap films useful for the packaging or palletizing of goods. The films disclosed include at least three layers having two outer layers and at least one intermediate layer placed between the outer layers. The intermediate layer is formed from a high pressure low density polyethylene resin and the outer layers are formed from a linear low density polyethylene copolymer such as ethylene copolymerized with a minor amount of at least one C.sub.4 to C.sub.10 alpha-olefin, such as hexene-1, octene-1, decene-1, 4-methyl-pentene-1 and butene-1. The force required to stretch the film may be controlled by the thickness of the intermediate laver relative to overall film thickness, the melt index of the intermediate laver resin, the molecular weight of the intermediate layer resin or a combination thereof.
As previously indicated, methods of stretch wrapping articles, containers, etc., are known. U.S. Pat. No. 3,986,611 describes a tension-wrapped palletized load obtained with a stretch wrap film possessing a cling additive.
U.S. Pat. No. 4,079,565 describes a stretch-wrapped package, process and apparatus which employs a stretch wrap polyethylene film.
U.S. Pat. No. 4,409,776 discloses a method and apparatus for packaging articles with a composite stretch wrap film, one surface of which is nonadhesive. The adhesive surface is obtained with an "adhesive film" such as one fabricated from ethylene-vinyl acetate copolymer, 1,2-polybutadiene or styrenebutadiene copolymer and the nonadhesive surface is obtained with a "nonadhesive film" such as one fabricated from a crystalline polyolefin, e.g., polyethylene, polypropylene or ethylene-propylene copolymer.
While it is apparent that a wide variety of stretch wrap films have been disclosed for use in packaging or palletizing operations, virtually all suffer from one or more notable deficiencies. Not all films known in the art Dossess good on-load stretched cling properties. Other films are deficient in their tensile properties, including the force required in the machine direction (MD) to stretch the film. Still, not all end use applications require the same film characteristics, necessitating the complex production of a broad range of films to suit these applications.
It is therefore an object of the present invention to provide a novel multilayered coextruded film which exhibits aood stretched cling and good machine direction stretch properties so as to provide utility in industrial pallet wrapping applications.
It is another object of the present invention to provide a novel multilayered coextruded stretch wrap film having good cling properties and high machine direction load force.
It is another object of the present invention to provide a multilayered coextruded stretch wrap film having high puncture and tear resistance.
It is another object of the present invention to provide a multilayered coextruded stretch wrap film having high resistance to tear when stretched in the transverse direction.
It is yet another object of the present invention to provide a process for the production of the novel films of this invention.
It is yet a further object to overcome the aforementioned problems in an effective and economical manner.
Other objects of the invention include the use of a stretch wrap film of the aforementioned characteristics in the overwrapping of a plurality of goods to provide a unitized packaged unit.
Further objects of this invention will become apparent from a reading of the specification and appended claims.